Abstract

We demonstrate that a single all-optical sensor based on laser diode self-mixing interferometry can monitor the independent displacement of individual portions of a surface. The experimental evidence was achieved using a metallic sample in a translatory motion while partly ablated by a ps-pulsed fiber laser. A model based on the Lang-Kobayashi approach gives an excellent explanation of the experimental results.

Left: Representative oscilloscope traces showing the time dependence of the signals detected by (a) the external photodiode (PDext), (b) the integrated photodiode PD1 and (c) the integrated photodiode PD2, during the laser percussion drilling of a 100 µm-thick stainless steel plate. The target displacement is controlled by a linear stepper motor stage moving at about 0.2 mm/sec. Relative distance between the collimating lens L1 and the diode emitting window: f1. The laser fluence was ~1.35 J/cm2. Inset: PD1 signal during the percussion drilling shown on an enlarged scale.

Representative oscilloscope traces showing the time dependence of the signals detected by: (a) the integrated photodiode PD1 and (c) the integrated photodiode PD2. (b) Zoom of PD1 signal during the ablation time. (d) Normalized power spectrum of the PD1 trace. The dashed line envelops the low and high-frequency peaks. The target displacement is controlled by a linear stepper-motor stage moving at about 0.2 mm/sec. Relative distance between the collimating lens L1 and the diode emitting window: f1 + 35 µm. The variable attenuator filters the optical feedback of 25% less than in Fig. 2. Laser fluence for the percussion drilling ~1.35 J/cm2.

Representative oscilloscope traces showing the time dependence of the signals detected by: (a) the integrated photodiode PD1 and (c) the integrated photodiode PD2. (b) Zoom of PD1 signal during the ablation time. (d) Normalized power spectrum of the PD1 trace. The target displacement is controlled by a linear stepper-motor stage moving at about 0.2 mm/sec. Relative distance between the collimating lens L1 and the diode emitting window: f1 + 35 µm. The variable attenuator filters the optical feedback of 50% less than in Fig. 2. Laser fluence for the percussion drilling ~1.35 J/cm2.